One cause of patient morbidity and mortality, idiosyncratic drug toxicity remains a serious safety concern in both clinical drug development and after market launch. These idiosyncratic drug reactions can lead to restricted use and even withdrawal from the market, which consequently results in higher development cost for the pharmaceutical industry. For example, troglitazone, benoxaprofen and zomepirac were withdrawn from the market shortly after their release due to unacceptable toxicity profiles.
Idiosyncratic drug reactions are a rare event that usually shows in a high degree of individual susceptibility. In addition, these reactions are usually not dose-dependent. Currently, there are no animal models that can be used to evaluate such reactions that exclusively occur in humans. Therefore, idiosyncratic drug toxicities cannot be effectively evaluated in preclinical studies, and are often unnoticed in clinical trials.
At present, the mechanisms of idiosyncratic drug reactions are not well understood. There is a substantial amount of evidence to suggest that chemically reactive metabolites are involved in idiosyncratic toxicities, especially for the liver. All drugs associated with idiosyncratic toxicity form reactive metabolites via various metabolic pathways mediated predominately by cytochrome P450s (CYPs), as well as by other oxidative enzymes such as peroxidases, cyclooxygenases and myeloperoxidases. It is hypothesized that drugs associated with such toxicities first undergo metabolic activation to generate toxic reactive metabolites that covalently bind to cellular proteins. These covalently modified proteins are immunogenic and thus trigger an immune response, resulting in idiosyncratic drug reactions. An alternative hypothesis states that covalent modifications of cellular proteins by reactive metabolites impair signal transduction cascades and vital functions of cells, leading to severe consequences observed in the clinic. Thus there remains a need for methods for identifying reactive metabolites.
Avery, Michael, J. (EP 1,150,120, Oct. 31, 2001) disclosed a high-throughput screening method for identifying drug candidates producing reactive metabolites. The method comprises incubating a drug candidate with a microsomal drug metabolizing enzyme system in the presence of glutathione and detecting glutathione conjugates formed therefrom using tanden mass spectrometry.
This method, however, will identify reactive metabolites as well as non-reactive components (including both unreactive metabolites and components of the reaction mixture) formed as a result of common response in mass spectrometry detection, thus resulting in false positives.
Thus, there remains a need for a method for detecting reactive metabolites which does not yield false positives.